This invention relates to high-temperature materials. More particularly, this invention relates to coatings to protect substrates in high-temperature, oxidative environments, such as, for example, gas turbine engines. This invention also relates to methods for protecting substrates in these environments.
In a gas turbine engine, compressed air is mixed with fuel in a combustor and ignited, generating a flow of hot combustion gases from the combustor to the turbine stages via a transition piece, also called a xe2x80x9cduct.xe2x80x9d Upon entering the turbine stage portion of the engine, the hot gas is driven through one or more turbine stages that extract energy from the gas, producing output power. Each turbine stage includes a stator nozzle having vanes which direct the combustion gases against a corresponding row of turbine blades extending radially outwardly from a supporting rotor disk. The vanes and blades are subject to substantial heat load, and, because the efficiency of a gas turbine engine is proportional to gas temperature, the continuous demand for efficiency translates to a demand for airfoils that are capable of withstanding higher temperatures for longer service times.
Gas turbine components such as, for example, vanes, blades, combustors, and transition pieces, are usually made of superalloys and are often cooled by means of internal air-cooling chambers and the addition of coatings, including thermal barrier coatings (TBC""s) and environmental coatings, to their external surfaces. The term xe2x80x9csuperalloyxe2x80x9d is usually intended to embrace iron (Fe)-, cobalt (Co)-, or nickel (Ni)-based alloys, which include one or more other elements including such non-limiting examples as aluminum (Al), tungsten (W), molybdenum (Mo), rhenium (Re), tantalum (Ta), and titanium (Ti). TBC""s comprise at least a layer of thermally insulating ceramic, often yttria-stabilized zirconia, and often include one or more layers of metal-based, oxidation-resistant materials (xe2x80x9cenvironmental coatingsxe2x80x9d) underlying the insulating ceramic for enhanced protection of the airfoil. Environmental coatings are also frequently used without a TBC ceramic topcoat.
Conventional coatings used on components exposed to the hot gases of combustion in gas turbine engines for both environmental protection and as bond coats in TBC systems include both diffusion aluminides and MCrAl(X) coatings. The term xe2x80x9caluminidesxe2x80x9d encompasses a wide variety of coatings comprising aluminide compounds of various chemical compositions. For example, nickel aluminide, NiAl, is often grown as an outer coating on a nickel-based superalloy by exposing the superalloy substrate to an aluminum-rich environment at elevated temperatures. The aluminum diffuses into the substrate and combines with the nickel to form a coating of NiAl on the outer surface. A platinum-containing nickel aluminide (Ptxe2x80x94NiAI) coating is often formed by electroplating platinum over the nickel-base substrate to a predetermined thickness, followed by exposing the platinum-coated substrate to an aluminum-rich environment at elevated temperatures. In addition to aluminide coatings, MCrAl(X) coatings, where M is at least one of Ni, Co, and Fe, and wherein X is at least one of yttrium (Y), tantalum (Ta), silicon (Si), hafnium (Hf), titanium (Ti), zirconium (Zr), boron (B), carbon (C), are commonly used as bondcoats for a TBC system and as environmental coatings. MCrAl(X) coatings are suitable for application by any of a number of processes, including plasma spraying, high-velocity oxy-fuel (HVOF) spraying, and physical and chemical vapor deposition, as non-limiting examples.
The coatings noted above comprise relatively high amounts of Al with respect to the superalloy substrate, while the superalloy substrate is relatively rich in certain elements, such as, for example, Co and refractory elements such as W, Re, Ta, Mo, and Ti, that are not present or are present in relatively small amounts in the coating. The concentration gradient in Al and other elements that results when the coating is deposited directly on the superalloy substrate causes solid-state diffusion to occur across the substrate/coating interface when the substrate is exposed to high temperatures, such as those found in gas turbine engines during service. The migration of elements across the interface alters the chemical composition and microstructure of both the coating and the substrate in regions adjacent to the interface (xe2x80x9cdiffusion zonesxe2x80x9d), resulting in generally deleterious effects on the properties of both the coating and the substrate. For example, migration of Al out of the diffusion zone of the coating detracts from its effectiveness as an oxidation-resistant material, while the accumulation of Al in the diffusion zone of the substrate converts the Y-Yxe2x80x2 superalloy structure into one with drastically reduced load-carrying capability. As time passes, and more material migrates across the interface, the size of the diffusion zones in both the coating and the substrate increases (xe2x80x9cdiffusion zone growthxe2x80x9d), leading to significant degradation in the performance of a coated superalloy component.
One solution to the problem of diffusion zone growth in gas turbine engine components is the use of a diffusion barrier layer, interposed between the substrate and the environmental coating, as set forth in U.S. Pat. No. 6,306,524. In part, ""524 sets forth certain alloys of ruthenium (Ru) with Ni, Co, Cr and mixtures thereof that can be effective as barriers to the diffusion of Al and other elements, thereby prolonging the life of coated gas turbine engine components by significantly the formation and growth of diffusion zones during service.
Although effective as diffusion barriers, some of the alloys set forth in ""524 lack the oxidation resistance needed for optimum performance in state-of-the-art gas turbine engines. Therefore, there is a need to provide material compositions and coating systems with high oxidation resistance and high resistance to solid-state diffusion of Al and other elements found in superalloys. Embodiments of the present invention address this need.
One embodiment is an alloy for use in a high-temperature, oxidative environment, comprising, in atomic percent: from about 20% to about 80% ruthenium (Ru), from about 2% to about 15% aluminum (Al), up to about 15% chromium (Cr); and the balance comprising at least one of nickel (Ni), cobalt (Co), and mixtures thereof.
A second embodiment is an alloy for use in a high-temperature, oxidative environment, comprising, in atomic percent: at least about 80% ruthenium (Ru), from about 2% to about 20% aluminum (Al), up to about 20% chromium (Cr), and the balance comprising at least one of nickel (Ni), cobalt (Co), and mixtures thereof.
A third embodiment is an alloy for use in a high-temperature, oxidative environment, comprising, in atomic percent: from about 60% to about 85% ruthenium (Ru), from about 15% to about 40% aluminum (Al), up to about 25% chromium (Cr), and up to about 25% of at least one of nickel (Ni), cobalt (Co), and mixtures thereof.
A fourth embodiment is a protective coating system for protecting an article from a high-temperature, oxidative environment. The protective coating system comprises a diffusion barrier layer comprising an alloy selected from the group (hereinafter referred to as xe2x80x9cthe composition groupxe2x80x9d) consisting of the following: an alloy comprising, in atomic percent, from about 20% to about 80% ruthenium (Ru), from about 2% to about 15% aluminum (Al), up to about 15% chromium (Cr), and the balance comprising at least one of nickel (Ni), cobalt (Co), and mixtures thereof; an alloy comprising, in atomic percent, at least about 80% Ru, from about 2% to about 20% Al, up to about 20% Cr, and the balance comprising at least one of Ni, Co, and mixtures thereof; an alloy comprising, in atomic percent, from about 60% to about 85% Ru, from about 15% to about 40% Al, up to about 25% Cr, and up to about 25% of at least one of Ni, Co, and mixtures thereof; and an alloy comprising, in atomic percent, from about 20% to about 45% Ru, from about 15% to about 40% Cr, from about 2% to about 50% Al, and the balance comprising at least one of Ni, Co, and mixtures thereof.
A fifth embodiment is an article for use in a high-temperature, oxidative environment. The article comprises a substrate and a diffusion barrier layer disposed on the substrate. The diffusion barrier layer comprises an alloy selected from the aforementioned composition group.
A sixth embodiment is a method for protecting an article from exposure to a high-temperature, oxidative environment. The method comprises providing a substrate, and disposing a diffusion barrier layer on said substrate. The diffusion barrier layer comprises an alloy selected from the aforementioned composition group.